<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>字符串大小写转换</title>
</head>
<body>
    


    <script>
        function toUpperCase(str) {
            return str.toUpperCase();
        }

        function toLowerCase(str) {
            return str.toLowerCase();
        }

        let originalStr = "wwwjb51.itcase.cn";
        let upperCaseStr = toUpperCase(originalStr);
        let lowerCaseStr = toLowerCase(originalStr);

        document.write("原字符串为：" + originalStr + "<br/>");
        document.write("大写形式：" + upperCaseStr + "<br/>");
        document.write("小写形式：" + lowerCaseStr + "<br/>");
    </script>
</body>
</html>